Method for fabricating superplastically formed/diffusion bonded structures

ABSTRACT

A method for fabricating superplastically formed/diffusion bonded structures wherein metal blanks of a titanium alloy are joined at selected areas by diffusion bonding and expanded superplastically to form a desired sandwich or integrally stiffened structure. In such method, the metal blanks are treated in selected areas with a &#34;stopoff&#34; material to prevent bonding at those areas during diffusion bonding and to permit forming or shaping at the same areas during superplastic forming. An improved stopoff compound is provided for this purpose, in the form of yttria of relatively coarse particle size, coarser than 5 microns, in a suitable volatilizable vehicle. Such stopoff compound is inert to reactive metals such as titanium at the high diffusion bonding temperatures, and permits relatively low breakthrough pressure-time product during superplastic forming, thereby preventing excessive strain or rupture of the metal through non-superplastic deformation.

BACKGROUND OF THE INVENTION

This invention relates to the fabrication of metallic sandwich andintegrally stiffened structures, and is particularly directed to amethod of making such structures by superplastic forming and diffusionbonding (SPF/DB), employing an improved composition specifically forfacilitating breakthrough prior to superplastic forming.

A number of alloys exhibit superplasticity and are capable of beingsubjected to superplastic forming to produce parts of predeterminedshapes. Superplasticity is the capability of a material to developunusually high tensile elongation with reduced tendency toward localnecking during deformation. However, this invention is particularlyconcerned with superplastic metals which are subject to contamination ofsurface integrity at forming temperatures. These are termed "reactive"metals. This includes alloys of titanium, zirconium and the refractorymetals.

Diffusion bonding refers to the solid-state, metallurgical joining ofsurfaces of similar or dissimilar metals by applying heat and pressurefor a time duration so as to effect intimate surface contact and causeco-mingling of atoms at the joint interface.

U.S. Pat. No. 3,927,817 discloses a method for fabrication of structuresin which metal blanks, preferably of a titanium alloy, are joined atselected areas by diffusion bonding at elevated temperatures andpressures, and then subjected to superplastic forming to form a desiredstructure. The metal blanks are first treated at selected areas with astopoff material, such as yttria, boron nitride, graphite, or alumina,to prevent bonding at such treated areas during diffusion bonding.During superplastic forming the metal blanks are expanded at the treated(unbonded) areas into contact with shaping members by increasing theinternal pressure, preferably with inert gas, thus forming an expandedstructure of a desired shape, essentially in a single operation.

Thus, after the bonds between adjacent metal blanks are formed duringdiffusion bonding, inert gas pressure, such as argon or helium, isapplied to the interior network to superplastically form the unbondedportions of the adjacent metal sheets. For such superplastic forming tooccur, gas must penetrate the entire interior network of unbonded(stopped off) areas. Initial flow of gas into an inlet, through theunbonded network and out the exit plumbing, is termed "breakthrough." Ifno breakthrough takes place, or if insufficient breakthrough takesplace, acceptable superplastic forming cannot occur. Breakthrough may becharacterized by the product of the time and pressure required for it tooccur. If a combination of high pressure and extended time is requiredto effect breakthrough, the parts may be ruptured or improperly formed,and scrapped.

Resistance to breakthrough results from the combined effects of: (1) thesmall cross-sectional area of the stopoff path; (2) the tortuosity andlength of the stopoff path; (3) the low gas permeability of the stopoffpath after bonding; and, (4) the resistance to overall bending of theunbonded, unsupported span of the sheet metal adjacent to the stopoffand to the tool cavity. Both the small cross-sectional area and thestopoff path length and tortuosity are fixed by design considerationsand are not good candidates for process control. However, thepermeability and the resistance to bending are both subject to controlas hereafter described.

During diffusion bonding, the stopoff layer is hot pressed by thebonding pressure and temperature and its permeability is significantlyreduced. Any measures which tend to resist the reduction of permeabilitywill result in an improved stopoff system. If stopoff were not present,the resistance of the unsupported span to bending is simply determinedby the material's strength properties and the geometric configuration ofthe span. For the small deflections necessary to establish the verymodest gas flows required for SPF/DB, the bending force, and thereforethe breakthrough pressure time requirement, would be quite small exceptfor the narrowest of spans. However, an appreciable additionalresistance to bending must be overcome which results from adhesive bondsof the metal sheet to the stopoff compound and cohesive bonds within thestopoff compound. Both of these bond types are created by compressionand sintering of the stopoff mass during the sustained pressure andtemperature conditions of the bonding cycle. Any attempt to alter theadhesive bonding characteristics of the stopoff to the reactive metalsheet would tend to involve foreign additions and is likely tocompromise the inert nature of the stopoff with respect to the sheet.However, cohesive bonding can be influenced by other than chemicalchanges. Hence it is necessary that the stopoff compound be one whichdoes not develop a high cohesive strength nor very low permeabilityduring diffusion bonding and thereby resist breakthrough prior to thesuperplastic forming operation.

In addition, it is necessary that the stopoff compound or composition beinert with respect to the reactive metal surface, so as to cause noformation of alpha-phase case on the metal surface. Alpha-case is thephenomenon whereby a reactive metal such as a titanium alloy thatdisplays a normal microstructure, consisting of a mixture of alpha-phase(hexagonal crystal structure) grains and beta-phase (body centeredcrystal structure) grains, is converted to all alpha at and near theexposed surface by the diffusion of oxygen and/or nitrogen into thealloy matrix. The conversion to alpha structure, which has a highinterstitial (oxygen and/or nitrogen) content, leads to a brittle layeron the surface which is particularly undesirable under fatigue loadingconditions because of a tendency to initiate fatigue cracks.

Of a variety of stopoff compounds investigated, including alumina,graphite, boron nitride, silicon nitride, and others, yttria was theonly one which was found to be sufficiently inert to reactive metalssuch as titanium and its alloys to avoid alpha case formation at thehigh temperatures encountered during diffusion bonding and superplasticforming. However, the use of yttria in its commercially available finepowder form often resulted in excessive breakthrough pressures andextended time periods to achieve breakthrough, with the attendantdangers and disadvantages noted above. Thus, in the above patent whichdiscloses use of such yttria as stopoff compound, breakthrough pressuresprior to superplastic forming are noted as ranging from 25 to 250 psi,and at such breakthrough pressures, time required to achievebreakthrough can require up to one or more hours. In terms of apressure-time product, the fine commercial grade yttria gave undesirablebreakthrough values usually in excess of 5000 psi-minutes.

Accordingly, one object of the invention is the provision of an improvedprocedure for producing SPF/DB structures.

Another object of the invention is the provision of a superplasticforming and diffusion bonding procedure for the above purpose, whichprovides a low breakthrough pressure-time product prior to superplasticforming, to permit forming to occur uniformly, avoid strain rates inexcess of the superplastic range, and avoid excessive local thinningan/or rupture of the metal sheets.

A still further object is the provision in the above noted superplasticforming and diffusion bonding process of a stopoff compound forapplication to preselected areas of the metal sheets, which is inert toreactive metals such as titanium and its alloys, and which also provideslow breakthrough pressure-time products to facilitate uniform andsuccessful forming during the subsequent superplastic forming operation.

SUMMARY OF THE INVENTION

The high breakthrough pressure-time product exhibited by the use ofyttria of the type heretofore commercially available as a stopoffcompound results from the diffusion bonding cycle pressure typically at300 psi, for one and one-half hours at 1700° F., for a Ti-6Al-4V alloy,which causes the yttria to develop low permeability and high adhesiveand cohesive strengths and thereby introduces a significant resistanceto breakthrough of inert gas prior to superplastic forming.

It has been found according to the invention that by controlling andreducing the degree of permeability loss and of cohesion, or the bondingof yttria particles to other yttria particles, satisfactorily lowbreakthrough pressure-time products can be achieved. Such control andreduction of the permeability loss and cohesive properties of the yttriastopoff compound can be achieved, according to the invention, byemploying yttria of significantly coarser average particle size than thefine commercially available yttria powder that has a maximum particlesize of approximately 2 microns. The use of yttria having asubstantially coarser particle size than the fine commercial gradeyttria, substantially reduces the cohesive properties of the yttria toreduce breakthrough pressure-time product, by decreasing the surfacearea and thus the sinterability of the yttria particles.

Similarly, the residual permeability after bonding under the sameconditions will be greater for coarse particles than for fine particles.Hence by employing yttria of relatively coarse particle size, as definedin greater detail hereinafter, superplastic forming and diffusionbonding operations for production of metallic sandwich structures, e.g.as described in above U.S. Pat. No. 3,927,817 (integrally stiffenedstructures are also referred to as sandwich structures in this patent),can be readily and successfully accomplished using relatively lowbreakthrough pressure-time products prior to superplastic forming,usually not in excess of about 100 psi-minutes, to produce properlyformed parts without defects or ruptures occurring during the subsequentsuperplastic forming operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood by reference to thedetailed description below of certain preferred embodiments taken inconnection with the accompanying drawings wherein:

FIG. 1 illustrates a metallic sheet having an area of stopoffcomposition or paint applied to selected surfaces;

FIG. 2 illustrates an assembly of the metallic sheet containing stopoffareas illustrated in FIG. 1 as the bottom sheet, with a top metal sheetplaced in contacct therewith, and the assembly inserted in a formingapparatus comprised of an upper and lower tool and subjected toselective diffusion bonding;

FIG. 2a is a section taken on line 2a--2a of FIG. 2; and

FIG. 3 illustrates forming the assembly of metal sheets in FIG. 2 into apredetermined shape by superplastic forming.

DETAILED DESCRIPTION OF THE INVENTION

The essential component of the present stopoff composition is yttria ofsufficiently coarse particle size to provide a low breakthroughpressure-time product prior to superplastic forming, as furtherdescribed below. Such yttria is in the form of a relatively coarsematerial having particles of a size larger or coarser than 5 microns andmost desirably with a substantial portion of the particles, e.g. 30-35%,coarser than 10 microns. This relatively coarse yttria material can beproduced by heating (sintering) in known manner the commerciallyavailable normally fine yttria powders which generally have a maximumparticle size of approximately 2 microns, to cause their agglomerationinto larger particle sizes, usually followed by grinding or comminutingthe agglomerated particles to the desired coarse size as noted above.

Although yttria of the above noted relatively coarse particle size isthe preferred essential stopoff component of the invention, theinvention concept is also applicable to any other compounds which, likeyttria, have a high free energy of formation and are thus inert toreactive metals at high temperatures and which in their normal finepowder form are highly sinterable and require excessive breakthroughpressure-time products, but which can be formed into materials ofsufficiently coarser particle size, as by sintering and agglomeration,to control and reduce the cohesiveness of the sintered mass resultingfrom the coarser particles during diffusion bonding, and provide a lowbreakthrough pressure-time product prior to superplastic forming asdescribed in greater detail hereinafter.

The essential component of the stopoff compound, e.g. the yttria ofrelatively coarse particle size, is incorporated into a vehicle orcarrier for the yttria, to form a liquid composition with the yttriaparticles suspended therein. The resulting composition, for example, canbe in the form of a silk screen paint which can be applied to the metalsheets or blanks by the usual silk screen procedure, or the stopoffcomposition can be formulated as a spray for spray application to themetal sheets or for any other useful method of selective paint or filmapplication. The vehicle for the yttria contains a binder to hold theyttria in position during bonding, but such binder must be sufficientlyvolatile to vaporize at temperatures substantially below those fordiffusion bonding, and leave an essentially pure yttria layer at thehigh diffusion bonding temperature, with substantially no organicresidue.

Accordingly, any organic resin in solid or liquid form and which has theabove characteristics can be employed as the binder for the yttria. Ithas been found, for example, that an acrylic resin is particularlysuitable as a binder, although other resins such as polyvinyl alcoholcan also be employed. The acrylic resin can be in the form of an acrylicmolding powder, the acrylic resin therein being, for example, polymethylmethacrylate.

There are also added to the organic binder various organic solvents forthe binder or resin component. These include, for example, esters suchas the low molecular weight alkyl acetates, e.g. butyl and ethyl acetatewherein the alkyl group contains up to four carbon atoms, ketons, suchas acetone, aromatic hydrocarbons such as benzene or toluene, andmixtures thereof. The combination comprised of an acrylic resin binderand an alkyl acetate such as butyl acetate or ethyl acetate, or mixturesof such acetates, as solvents is particularly suitable as liquid vehiclefor the yttria stopoff compound. However, the liquid vehicle can becomprised essentially of binder alone, and in the absence of solvent orthinner, where the binder is a liquid resin. Where a solvent is employedin combination with a binder, such solvent will also volatilize with thebinder during heating of such liquid vehicle containing the yttria todiffusion bonding temperatures.

The proportion of yttria or other suitable stopoff compound, accordingto the invention, present in the liquid stopoff composition can rangefrom about 100 to about 4,000 grams, usually about 300 to about 3500grams, of yttria or other suitable stopoff compound, per liter of totalorganic liquid vehicle, including binder and solvent, where the latteris present. For example, the more dilute liquid compositions have aconsistency compatible with spraying, while the more concentrated liquidcompositions are utilized for silkscreening and brushing.

The stopoff compositions of the invention are usually produced by ballmilling the components, e.g. for 1 to 2 hours, to assure good mixing andpigment (yttria) suspension, though other methods of mixing can be used.

The following are examples of stopoff compositions according to theinvention:

EXAMPLE 1

    ______________________________________                                        Silk Screen Paint                                                             Components            grams     ml                                            ______________________________________                                        Acrylic molding powder                                                                              50                                                      Butyl acetate                   500                                           Acetone                         250                                           Yttria (100% of the particles                                                                       1,000                                                   coarser than 5 microns, with                                                  30-35% coarser than 10 microns)                                               ______________________________________                                    

The acrylic molding powder is believed to comprise essentiallypolymethyl methacrylate.

The above components are mixed in a ball mill and 250 ml acetone isadded to the ball milled sludge for thinning.

EXAMPLE 2

    ______________________________________                                        Spray Composition                                                             Components           grams     ml                                             ______________________________________                                        Acrylic molding powder                                                                             50                                                       (as in Example 1)                                                             Ethyl acetate                  750                                            Butyl acetate                  250                                            Yttria (as in Example 1)                                                                           1,000                                                    ______________________________________                                    

The above components are mixed in a ball mill.

A mixture of 575 ml ethyl acetate and 125 ml butyl acetate as thinner,is added to the above ball milled composition.

Now referring to the drawing, in practicing the present invention, thestopoff paint of Example 1 or the stopoff composition of Example 2 isapplied to selected areas or strips 12 of a clean titanium alloy sheetor blank 10, to prevent bonding in those areas. While other reactivemetals could be used, this invention has been found to be particularlyadvantageous for use with a titanium alloy, such as Ti-6Al-4V. If thestopoff paint of Example 1 is used, it is applied by silk screening thepaint by conventional silk screening procedure, e.g. in the form ofstrips, as indicated at 14, in any suitable configuration, over thepreselected areas 12. If the stopoff composition of Example 2 isemployed, it is sprayed by suitable means onto the stopoff areas 12,employing suitable masking to protect those areas 16 of the titaniumalloy sheet which are to be bonded, including the area between thestrips 14 and the peripheral areas around the strips. Grooves 11 and 13are provided in sheet 10 to allow for passage of pressurizing fluid tothe stopoff network.

Referring to FIGS. 2 and 2a of the drawing, with the titanium alloysheet 10 thus treated with the yttria stopoff composition according tothe invention, utilized as the bottom sheet (it could alternately be thetop sheet), a second clean titanium alloy sheet or blank 18 is placed asthe top sheet, over the stopoff-treated sheet 10, with the sheet 18 incontact with the strips of stopoff composition 14, applied to the bottomsheet 10.

The resulting assembly 20 of titanium alloy sheets 10 and 18, with thestrips of stopoff composition 14 therebetween, are placed in a formingapparatus, generally indicated at 22, and having an upper tool 24 and alower tool 26. The lower tool contains a plurality, here shown as two,die cavities 28. A gas inlet 30 is provided in communication with groove11 for introducing gas into the stack or assembly 20 and a gas outlet 32in communication with groove 13 is provided for exiting gas from theassembly.

Diffusion bonding is accomplished by heating the sheet assembly or stock20 to suitable diffusion bonding temperature while maintaining theassembly in an inert gas atmosphere, by passage, e.g. of helium orargon, through the stack via gas inlet and outlet 30 and 32 and intocavities 28 (through conduits not shown). When the metal sheets of thestack 20 have been heated to a suitable diffusion bonding temperature,pressure is applied to the stack by application of gas pressure incavities 28 in the lower tool 26 against the lower surface of sheet 10.Pressure could alternately or in addition be provided by tooling 25 and26. Diffusion bonding temperatures can vary from 1450° F. to about 1850°F., e.g. about 1700° F. for 6Al-4V titanium alloy, and bonding pressurecan vary from about 100 psi to about 2,000 psi or more, usually fromabout 150 psi to about 600 psi. The time for diffusion bonding can rangefrom as little as 30 minutes up to about 15 hours.

In a typical operation, a diffusion bonding cycle of 300 psi pressureapplied for 11/2 hours at 1700° F. was used to bond the unstoppedoffareas of the 6Al-4V titanium alloy, as indicated at 16, between thestopoff strips 14. As previously noted, constant purge of inert gas wasmaintained through the gas inlet and outlet 30 and 32 during the heat-upand diffusion bonding cycles to flush the volatilized organic componentsof the stopoff composition (strips 14) from the assembly, as well as toavoid any sheet contamination by air, leaving a layer of yttria in theareas 12.

Now referring to FIG. 3 of the drawing, after diffusion bonding, gaspressure at inlet 30 and between the sheets 10 and 18 and in stopoffareas 12 is increased to initiate complete gas communication from inlet30 through the stopoff pattern area to the exit 32, prior tosuperplastic forming. It is desirable to achieve this gas communicationor breakthrough with nil to minimal forming of the part. This is so thatthe forming may be accomplished in a completely controlled manner. Ifthe pressure-time product used to achieve breakthrough is a substantialportion of or even exceeds that required for the initial (and critical)part of the forming cycle, the forming process will be out of controland result in unsatisfactory parts. It should be noted that thetemperatures used for breakthrough are normally in the same range asthose for diffusion bonding and superplastic forming, i.e. 1650°-1750°F. for Ti-6Al-4V. For this reason, good breakthrough characteristics,best described by pressure-time product, are essential to the successfulpractice of SPF/DB.

The effectivity of the present invention is clearly indicated by thedata presented in Table I. This data results from special breakthroughtest panels in which several independent stopoff paths are individuallyplumbed so that when subjected to the same pressurization conditions,the breakthrough characteristics of each path may be independentlyevaluated. It is seen that the pressure-time product for coarse yttriain Table I ranges from about 3 to about 70 psi-minutes and is two ordersof magnitude less than that of the regular fine yttria, previously theonly form available. For further comparison, data on boron nitride as astopoff are included. The average particle size of the boron nitride inthese tests was between one and two microns. It is seen that boronnitride gives excellent pressure-time products but unlike the yttriacreates alpha-case contamination on the titanium surface. It should benoted that coarse yttria and boron nitride breakthrough pressure-timeproducts are of the same order of magnitude.

                  TABLE I                                                         ______________________________________                                        Comparison of Breaktrhough Behavior                                                                            Breakthrough                                           Pressure at                                                                              Time to     Pressure-Time                                          Breakthrough                                                                             Breakthrough                                                                              Product                                      Stopoff System                                                                          PSI        Minutes     PSI-Minutes                                  ______________________________________                                        Regular Yttria                                                                          20         50          1,000                                        Regular Yttria                                                                          50         105         5,250                                        Boron Nitride                                                                           4          3           12                                           Boron Nitride                                                                           6          2           12                                           Boron Nitride                                                                           40         .08         3                                            Coarse Yttria                                                                           64         .5          32                                           Coarse Yttria                                                                           4          2           8                                            Coarse Yttria                                                                           6          5           30                                           Coarse Yttria                                                                           4          3           12                                           Coarse Yttria                                                                           14         5           70                                           Coarse Yttria                                                                           40         .08         3                                            ______________________________________                                    

Another indication of the effectivity of the present invention is toconsider the pressure-time product involved in the critical part of asuperplastic forming cycle. While these cycles vary over a wide rangeaccording to the geometry of forming and the material properties andthickness, it might be generalized that this critical portion of thecycle may involve pressure-time products of 100 to 1000 psi-minutes.Thus, it is seen that the 1000 to 5000 psi-minutes for normal yttria isintolerable while the 12-70 psi-minutes for the coarse yttria isacceptable. Based on the above, the definition of a low breakthroughpressure-time product is, essentially, that product which permitsforming to occur uniformly, avoids strain rates in excess of thesuperplastic range and avoids local thinning and/or rupture of the metalsheets in all cases. A low breakthrough pressure-time product isgenerally considered to be less than 100 psi-minutes.

After breakthrough, the stack 20 is superplastically formed undersuitable temperature and pressure conditions (such as 1650°-1750° F. and25-250 psi for Ti-6Al-4V) by stretching of the lower sheet at theunbonded areas 12 previously covered by the present stopoff compositioninto contact with the configured surfaces 28 of lower tool 26 to formthe expanded portion 36 of assembly 20.

The superplastic forming and diffusion bonding operation, and theapparatus for carrying out such procedure, are described in detail inabove U.S. Pat. No. 3,927,817. Such disclosure is incorporated herein byreference, but forms no part of the present invention.

It was noted that when employing the stopoff compositions of theinvention, such as illustrated in Examples 1 and 2 above, and attemperature of 1700° F., for producing a structure as illustrated inFIG. 3, breakthrough pressure-time product values of 3-70 psi-minuteswere obtained prior to superplastic forming, and the expanded portions36 of the assembly under these conditions were found to be uniform inthickness, with no local necking, strains or ruptures present.

On the other hand, when employing stopoff paints or formulations havingthe same composition as Examples 1 and 2 above, but employing thereinthe commercially available yttria powder having a maximum particle sizeof about 2 microns, under the same conditions of diffusion bonding andsuperplastic forming temperatures, and for producing sandwich structuresof the same shape, the breakthrough pressure-time product wassubstantially higher, e.g. approximately 5000 psi-minutes, resulting inexpanded portions such as 36 of the assembly, which were non-uniform,and showing necking or excessively thin portions, and substantial localstrain.

From the foregoing, it is seen that the invention provides an improvedprocedure for superplastic forming and diffusion bonding of certainmetals, particularly titanium alloys, employing a stopoff composition toprevent diffusion bonding in preselected areas, containing as essentialcomponent, yttria of relatively coarse particle size, in excess of 5microns, resulting in the provision of relatively low breakthroughpressure-time product for breakthrough prior to superplastic forming,and the production of metallic sandwich and integrally stiffenedstructures of good uniformity and absence of local strain and rupturesin those portions expanded during superplastic forming.

Since various changes and modifications of the invention will occur toand can be made readily by those skilled in the art without departingfrom the invention concept, the invention is not to be taken as limitedexcept by the scope of the appended claims.

What is claimed is:
 1. In the process for making a structure formed ofat least two metallic workpieces of a reactive metal including the stepsof treating said workpieces at selected areas with a stop-offcomposition to prevent bonding at said selected areas, placing said atleast two workpieces into contact with each other, maintaining saidworkpieces under coordinated temperature-pressure-time durationconditions to produce diffusion bonding of said workpieces atpreselected areas, and applying gas pressure between the contactingsurfaces of said at least two workpieces to cause breakthrough at saidselected areas and superplastic forming of at least one of saidworkpieces to stretch substantially in excess of its original surfacearea; the improvement which comprises utilizing a stop-off compositioncontaining as an essential component yttria, having a particle sizegreater than 5 microns, and an inert organic liquid vehicle for saidcompound, said organic vehicle comprising a binder capable ofvolatilizing and leaving substantially no organic residue at elevatedtemperature below the diffusion bonding temperature of said reactivemetal.
 2. The process as defined in claim 1 also including utilizing abreakthrough pressure-time product sufficiently low to produce astructure containing expanded portions of uniform cross-section andsubstantially free of local strains and ruptures.
 3. The process asdefined in claim 2 wherein said reactive metal is a titanium alloy. 4.The process as defined in claim 3 wherein the temperature is maintainedwithin a range of about 1,650° to about 1,750° F. during said diffusionbonding and said superplastic forming, and said pressure during saiddiffusion bonding is between about 150 and about 600 psi, and saidbreakthrough pressure-time product is not in excess of about 100psi-minutes.
 5. The process as defined in claim 4 wherein saidbreakthrough pressure-time product ranges from about 3 to about 70psi-minutes.
 6. The process as defined in claim 3 wherein saidbreakthrough pressure-time product is not in excess of about 100psi-minutes.
 7. The process as defined in claim 3 wherein saidbreakthrough pressure-time product is less than 10% of that resultingfrom the use of yttria having a maximum particle size of about 2microns.
 8. The process as defined in claim 1 wherein about 30 to about35% of the yttria particles are of a size greater than 10 microns. 9.The process as defined in claim 8, said composition being in the form ofa silk screen paint, said organic vehicle comprising acrylic moldingpowder as binder, and including butyl acetate and acetone, as solventsfor said acrylic molding powder, employing a range from about 300 toabout 3,500 grams of said yttria per liter of said organic vehicle. 10.The process as defined in claim 8, said composition being in the form ofa spray composition, said organic vehicle comprising acrylic moldingpowder, and including ethyl acetate and butyl acetate as solvents forsaid acrylic molding powder, employing a range from about 300 to about3,500 grams of said yttria per liter of said organic vehicle.
 11. Theprocess as defined in claim 1 employing a range from about 100 to about4,000 grams of said yttria per liter of said organic vehicle.
 12. Theprocess as defined in claim 1 wherein said organic vehicle comprises aresin binder.
 13. The process as defined in claim 12, said resin binderbeing an acrylic resin.
 14. The process as defined in claim 12, saidvehicle also including a solvent for said resin selected from the groupconsisting of esters, ketones, aromatic hydrocarbons, and mixturesthereof.
 15. The process as defined in claim 1 wherein said liquidvehicle comprises an acrylic resin binder and including a low molecularweight alkyl acetate, wherein said alkyl group contains up to fourcarbon atoms, as solvent for said resin binder.
 16. The process asdefined in claim 15 wherein said alkyl acetate is selected from thegroup consisting of ethyl acetate, butyl acetate, and mixtures thereof.